Much attention and effort has been applied in recent years to the production of fuels and chemicals, primarily ethanol, from cellulosic feedstocks, such as agricultural wastes and forestry wastes, due to their low cost and wide availability. These agricultural and forestry wastes are typically burned and landfilled. Thus, using these cellulosic feedstocks for ethanol production offers an attractive alternative to disposal.
The first chemical processing step for converting cellulosic feedstock to ethanol or other fermentation products usually involves pretreatment of the feedstock. The purpose of the pretreatment is to increase the cellulose surface area, with limited conversion of the cellulose to glucose. Pretreatment of the feedstock can be achieved using an acid pretreatment conducted under conditions that hydrolyse the hemicellulose component of the feedstock, followed by enzymatic hydrolysis of the cellulose remaining in the pretreated cellulosic feedstock with cellulase enzymes. Acid pretreatment typically hydrolyses the hemicellulose component of the feedstock to yield xylose, glucose, galactose, mannose and arabinose and this is thought to improve the accessibility of the cellulose to cellulase enzymes.
In one type of acid pretreatment process, the pressure produced by the steam is brought down rapidly with explosive decompression, which is known as steam explosion. Foody (U.S. Pat. No. 4,461,648) describes the equipment and conditions used in steam explosion pretreatment. Steam explosion with sulfuric acid added to achieve a pH of 0.4 to 2 produces pretreated material that is uniform and requires less cellulase enzyme to hydrolyze cellulose than other pretreatment processes.
Other pretreatment methods have been described in the literature including alkali pretreatment and mechanical pretreatment. Examples of alkaline pretreatment processes include ammonia fiber expansion (AFEX) and dilute ammonia pretreatment. According to the AFEX process, the cellulosic biomass is contacted with ammonia or ammonium hydroxide, which is typically concentrated, in a pressure vessel. Dilute ammonia pretreatment utilizes more dilute solutions of ammonia or ammonium hydroxide than AFEX.
Examples of mechanical pretreatment processes include those disclosed in US 2001/0012583 and in U.S. Pat. No. 5,498,766, which is discussed below. According to these processes, feedstock slurries are fed to a high shear device that comprises multiple concentric rings mounted on a rotor in a chamber. Slurry enters the center of the device and is forced out radially through gaps situated between teeth present in the rings. As the slurry passes through gaps and teeth in the rings, this introduces intense shear and cavitation in the feedstock. As set forth therein, this action increases the surface area of the substrate and disrupts its fiber structure.
The cellulase enzymes utilized to hydrolyze the cellulose to glucose include a mix of enzymes including exo-cellobiohydrolases (CBH), endoglucanases (EG) and beta-glucosidases. The CBH and EG enzymes catalyze the hydrolysis of the cellulose (β-1,4-D-glucan linkages). The CBH enzymes, CBHI and CBHII, act on the ends of the glucose polymers in cellulose microfibrils and liberate cellobiose, while the EG enzymes act at random locations on the cellulose. Together, the cellulase enzymes hydrolyze cellulose to cellobiose, which, in turn, is hydrolyzed to glucose by beta-glucosidase (beta-G). Enzymatic hydrolysis is typically conducted in one or more dilute mixed batch reactors under controlled pH, temperature and mixing conditions.
The fermentation to produce ethanol from the glucose is typically carried out with a Saccharomyces spp. strain. Recovery of the ethanol is achieved by distillation and the ethanol may be further concentrated by molecular sieves.
The addition of water to the incoming feedstock or the pretreated feedstock to form a slurry facilitates the transportation and mechanical handling of the cellulosic feedstock. The slurry consists of cellulosic feedstock or pretreated feedstock pieces or particles in water. In many lignocellulosic conversion processes described in the prior art to produce fermentable sugar, the solids content, measured as undissolved solids (referred to herein as “UDS”), is between 5 and 12 wt %. Such slurries are typically referred to as “medium consistency slurries”. The consistency of the aqueous slurry, expressed as the undissolved solids concentration (UDS), may be determined by the UDS procedure described in the examples.
However, for lignocellulosic conversion processes to be more economical, it would be desirable for the slurries to contain a higher undissolved solids content. The processing of slurries containing high solids content has numerous advantages in various stages of the process. For example, during chemical pretreatment, the lower water content in the incoming slurry requires less steam for the heat-up, as well as chemical. During enzymatic hydrolysis, the volumetric efficiency of the process is improved at high solids content. Furthermore, at high solids content, the hydrolysis product will contain a high concentration of fermentable sugars, which improves productivity.
Moreover, reduced volumes of water in the slurry result in reductions in equipment size, which, in turn, reduces capital cost. A further advantage of using high solids consistency slurries is that the amount of water supplied to the plant can be significantly reduced. Water usage adds significant expense to the process, especially in arid climates. Reducing the water requirements for the process would be a major step in making the process more economically viable.
Despite the foregoing advantages associated with utilizing high consistency slurries, the processing of such slurries downstream of pretreatment can pose problems, particularly during the enzymatic hydrolysis of cellulose. One problem that the inventor has identified is that the use of standard equipment to mix cellulase enzyme with pretreated feedstock slurry prior to enzymatic hydrolysis is not effective when the undissolved solids content of the pretreated slurry is high. Such mixing steps conducted prior to enzymatic hydrolysis are required in order to ensure that the cellulase enzyme is adequately dispersed in the pretreated feedstock slurry prior to commencement of enzymatic hydrolysis. A typical equipment configuration is a hydrolysis make-up tank, wherein enzyme and alkali are combined prior to entry of the slurry into the hydrolysis reactor. However, in order for a conventional mix tank to effectively disperse the enzyme in highly viscous slurry, a very large power input is required. This is because the rheological properties of such slurries suggest that the slurry will exhibit semi-solid type behaviour at low stress. Such rheological properties will result in high power requirements, which will significantly increase the operating costs of the hydrolysis stage of the process.
There has been much effort in the development of methods to hydrolyze cellulose to glucose using cellulase enzymes, much of which has focused on dilute systems. U.S. Pat. No. 5,248,484 discloses conducting enzymatic hydrolysis of cellulose in an agitated hydrolyzer that contains an internal stirring device. A side stream is withdrawn from the reactor and sent to an attritor that produces a high shear field for causing attrition or size reduction of the solid particulate. The stream exiting the attritor is then re-circulated back to the agitated hydrolyzer. Such a configuration exposes new substrate surface area to the enzyme so as to increase reaction efficiency. U.S. Pat. No. 5,508,183 discloses a similar equipment configuration for achieving enzymatic hydrolysis of cellulose.
However, the foregoing patents (U.S. Pat. Nos. 5,248,484 and 5,508,183) do not address the problems relating to hydrolyzing cellulose in high consistency systems, specifically the problems associated with introduction of cellulase enzyme to thick slurries. Moreover, the reactor systems disclosed therein would likely not be economically feasible for hydrolyzing cellulose in high consistency slurries. For instance, in order for such stirred reactors to mix a highly viscous slurry effectively during enzymatic hydrolysis, a very large power input would be required.
WO 2009/045651 discloses a fed batch reactor system including multiple size reduction steps and mixing to maintain thorough mixing of high consistency biomass in a vertical, agitated tank. Biomass is introduced into a vertical reactor tank equipped with an overhead agitator system such as a motor and shaft with one or more impellers. A mixable slurry is introduced into the reactor. For slurries without adequate levels of water, liquid is added prior to loading in order to sustain mixing under action of the agitator. The biomass slurry is then reacted under suitable conditions. An additional portion of pretreated biomass is added to the reactor to produce a higher solids biomass slurry as the slurry becomes less viscous as hydrolysis proceeds. Mixability of the slurry is monitored and biomass addition is controlled to maintain thorough mixing. However, the process does not address addition of enzyme to a high consistency slurry, but rather maintaining a high solids content in the reactor so as to achieve high sugar concentrations.
U.S. Pat. No. 4,409,329 discloses contacting an aqueous slurry comprising from 3-20 wt % solid cellulose containing stock with a cellulase enzyme, wherein the contacting occurs in the presence of shear through the reaction zone in a hydrolysis vessel. The vessel contains a concentric shaft which supports a number of perforated rotor blades which alternate with stator blades affixed to the walls of the vessel. However, such a hydrolysis vessel would require a high power input in order to mix high consistency slurries.
U.S. Pat. No. 5,498,766 (supra) discloses a pretreatment stage during which the biomass is shattered, shredded and disintegrated, so as to explode the fibers and rip them apart. It is reported that the resulting fibers exhibit extensive internal decrystallization due to microcavitation and shearing. As described previously, this decrystallization is carried out with a high-frequency, rotor-stator dispersion device having concentric, toothed rings within a chamber. Feedstock enters the center of the device and passes through gaps in the toothed rings due to the centrifugal force exerted by the device. The purpose of the shattering, shearing and disintegration step is to disrupt the lignin bonding to the cellulose and possibly the cellulose bonding to hemicellulose. This renders the cellulose material more available for hydrolysis by cellulase. Cellulase may be added to the slurry before or after exposure to the fiber explosion stage.
Other literature suggests that the activity of cellulases decreases with increasing shear rate or with vigorous mixing. Studies have shown that high shear or prolonged exposure of cellulases to shear during hydrolysis can cause the enzymes to denature (see for example, Cao and Tan, 2004, Journal of Macromolecular Science B43(6):1115-1121).
There is a need for more efficient and cost effective processes for hydrolyzing cellulose to glucose in high consistency slurries. In particular, there is a need in the art to further reduce capital and operating costs associated with such a process so as to make it more commercially feasible.